One does not have to be a scientist in this case to make a decision, no matter it is scientific sense, legal sense, or just gut feeling. The open letter is meant to express this feeling in the public, that's all.

If you feel that you cannot make a decision, don't sign it. That's perfectly fine. But it isn't fair to say no one except expert in that area can make a judgment on this case, especially when at least some of the issues here do not require such expertise.